Impedance box



Oct. l, L, BEHR 2,015,651

IMPEDANCE BOX Filed April 20, 1932 2 Sheets-Sheet 1 .fwenor Magcom 2 WO/RW Oct. l, 1935.

L. BEHR IMPEDANCE BOX Filed April 20, 1932 v 2 Sheets-Sheet 2 /NNER Row l aF co/vmczs l Gara? Row 0F CONTACTS l Top Sfar/olv 'olvfcr/av INNER' Ra w l 0F Co/vrAcrS cuff@ Row 0F CONTACIS Patented Oct. l," 1,935

IMPEDANCE BOX Leo Behr, Philadelphia,

Pa., assigner to Leeds Northrup Company, Philadelphia., ration of Pennsylvania Pa., a. corpo..

Application-April 20, 1932, Serial No. 606,365

7 Claims.

My invention relates to impedance boxes, and particularly to resistance boxes used in precision I measuring'instruments or systems, and particucluding brushes, for connecting the selected resistors of the decades to each other and to the box terminals are all of iixed length and posi` tion.

My invention also resides .in the features of construction, arrangement, and combination hereinafter described and claimed.

For an understanding of my invention, and for illustration of several forms it may take, reference is to be had ,to the accompanying drawings n which:

Fig. 1 diagrammatically illustrates a measuring circuit utilizing a common form of resistance box.

Fig. 2 is a diagram of the same circuit utilizing my new resistance box.

Fig. 3 is a bottom plan view of a six-decade box constructed in accordance with my invention.

Fig. 4 is a side elevation of Fig. 3.

Fig. 5 is a top plan View of the box of Fig. 3.

Fig. 6 illustrates another type of measuring circuit utilizing a modcation of my invention.

Fig. '7 in perspective, illustrates an attenuator resistance box constructed in accordance with my invention.

Figs. 8 and 9 illustrate vertical sectional and perspective views, respectively, of a contact brush used in connection with the structures of Figs. 2 5.

For a. better understanding of my invention, the errors introduced by use of the old style resistance boxes, particularly in alternating current measurements, are first discussed.

Referring to Fig. l, which illustrates a system for determining the value of an unknown resistance by the substitution method, the magnitude of the unknown resistance X is determined by iirst setting the contact arms I and 2 of the resistance box B to their zero positions, and ad- (Cl. 20L-48) justing the capacity C until the meter A indicates that the circuit is in resonance. The resistance X is then excluded from circuit, as by replacing it with a bar or strap of negligible resistance, and the contact arms l and 2, of the re sistance box B manipulated until the same reading of meter A is obtained. The value of resistance X is supposedly equal to the resistance introduced into the loop circuit by the movement of either or both of the contact arms I and 2 1 from their zero position.

However, with the box construction shown, the readings are not accurate because the change in positions of the arms I and 2 does more than insert resistance into the loop comprised of the 1 condenser C, ammeter A, the transformer secondary S, and conductors 3, 4, 5, 6, and 1. The change in setting also varies the inductance and/or capacity as well for reasons now given, and the error is diierent for different settings and for different frequencies of the source il. With the contact arms I and 2 in their zero position, the length of the loop between the terminals 8 and 9 of the resistance box is equal to the length of conductors 5, 6 and 'I plus the length of 25 the contact arms I and 2. This path Within the box possesses a certain inductance -Which is one of the factors determining the current through the ammeter A. Further the resistances rI-ril) associated with contact I, are always connected to one another by the conductors I0, and the same is true of resistance RI-RIO associated with contact 2 so that resistances not included in the circuit nevertheless exhibit capacitative effects which' affect the reading of meter A. 35

Briefly, for every different setting of the contacts I and 2, the box not only introduces a'known resistance into the measuring circuit, but also introduces spurious unknown and uncompensated inductance and capacity variations.

40 For example, with the contact arms in the position shown, the inductance between the terminals 8 and 9 of the box, and consequently of the entire loop circuit including condenser C, inductance S, conductors 3 and 4, etc., is increased by the inclusion of the several conductors I0 inter-connecting the resistors rI to 15, and RI to R4. The undesired and unavoidably interjected change in inductance is different for every combination of settings of the contact arms, and moreover the eiect 5 of these many inductance changes upon the ammeter reading depends upon the frequency with which the measurements are being made. lf the frequency of current generated or supplied by the source I I is high, for example 10,000 to 50,000 55 cycles per second, or higher, the change of inductance eiiected by the change in setting of arms I and,2, has all the more effect upon the current through the ammeter, so that the calibrations of the box cannot be relied uponas giving the true value of resistance X. The matter is more complicated because of the fact that for the diflerent settings of the resistance arms, the number ot inactive resistances, and their capacity eiiects, also vary.

Briefly, with the prior type of resistance boxes. it is not possible to obtain accurate measurements in alternating current systems, particularly when the frequencies are high, because the spurious inductance and capacity o! the internal circuit of the box is not constant but diierent for every di!- ferent setting.

Referring to Fig. 2, the current path within the box BI between its terminals l and! is always oi nxed length and position. The resistance ko to kin are mounted upon a movable support I2 and each is provided at its opposite ends with individual contacts I3 and Il. Each resistance is electrically independent of every other. As the support I2 is moved to the right, Fig. 2, resistors o! increasing magnitude are individually and seiectively included in circuit between the iixed brushes I5 and II. Similarly resistances K to Km are mounted upon a common support I1 which is movable to include the resistors individually and selectiveLv in circuit between the ilxed brushes 25 and Il which successively engage contacts Il and Il. The conductor I9 between brushes Il and Il is o! fixed length and position. It is apparent that with this arrangement, unlike Fig. 1, the circuit conilguration is not changed for the diierent settings oi the resistance box; the spurious inductance and capacity o! the box. and of the circuit as a whole, remain substantially constant, and only the resistance in circuit is changed. Consequently, with the box oi Fig. 2, the readings are very accurate for all settings and throughout a substantial range of frequencies.

When the supports I2 and I1 are in their zero position, the conductors ko and Kn oi negligible resistance are connected between the pairs oi stationary contacts IB-I and 25-II. When the supports are moved to any other position the symmetry o! the arrangement is not modiied by inclusion'oi any additional conductors, as in the box of Fig. 1, nor is there any change in capacity eects due to unused resistors hanging on to the resistances in circuit. Only one resistance oi each section is in circuit at a time and it is not in any way electrically connected to any other resistance o! that section.

The individual resistors are preferably of the low time constant type disclosed and claimed in copending applications Serial No. 586,518 and Serial No. 590,334, iiled January 4, 1932, and February 2, 1932, respectively which have matured as Patents 1,972,499 and 1,972,720, respectively.

It is understood of course, that the values of resistance kl to kII and KI to KII in accordance with usual practice, are selected to afford a wide range in values. with small steps of change, tor example, the resistances kl to kI may increase by .1 ohm steps from .i ohm to 1 ohm, and the resistances KI to Kil increase by l ohm steps from 1 to 10 ohms. Therefore, with this arrangement, anyreaistancevalue fromzeroto 11 ohmsmaybe obtained in .1 ohm steps. In other words, there are over one hundred diierent resistance settings. The same is true in the box shown in Fig. 1, but with that arrangement, there is also concurrently enected as many different imdesired inductance and capacity changes whereas in the arrangement shown in Fig. 2 the inductance and capacity remain substantially the same for all settings and only the resistance is changed. I

- to the resistance box should remain in nxed position, at least for any series of measurements, to preclude any change in the inductance and/or capacity of the entire circuit.

The box may, oi course, be used in direct curn rent measuring circuits so that its use is universal;

i. e., it may be used either in direct current measuring systems or in alternating current systems utilizing widely difiering frequencies.

'Ihe invention is all the more important as the g5 number of box groups, or decades, is increased, for the number or possible variations oi inductance and capacity iollow the laws of permutations and combinations. For example, with a sixdecade box, such as shown in Figs. 3 to 5, the 30 number of possible different inductances and capacity changes introduced is incredibly high. for example, of the order of one million. It is apparent that it is practically impossible to calibrate a resistance box to take into account all 35 oi' the changes ot inductance and capacity ier all the diiierent possible combinations oi settings, even at one frequency, much less for a wide range oi' frequencies. The simple arrangement shown in Fig. 2 is capable of being expanded for use with any number ot resistance box groups or decades and avoids any need for the attempt to consider the effects ot numerous changes in inductance and capacity.

Referring particularly to Fig. 4, the resistances or each group are mounted upon a rotatable sup- 5 port or drum 2l, corresponding generally to the supports I2 or I1 of Fig. 2. The resistances R. of each group are preferably arranged with their axes parallel to the shaft 2| upon which the drum is mounted. The series o: contacts la 5 angularly spaced at one end o! the drum, are connected to the corresponding ends o! the resistance R, while the contacts Il angularly mounted uponl the other end of the drum, are individually connected to the other ends of the 55 resistances. As diagrammatically shown in Fig.

2, each resistance of a group or decade is provided with a pair of contacts, and is electrically independent of every other resistance of that section.

A ilxed contact or brush Il, fastened for ex- V ample, to panel 23 is adapted to be successively engaged by the lower set of contacts Il as the first drum is rotated, while the brush I0, held in fixed Aposition upon the supporting member 2l 65 extending to the rear of panel 23, is adapted successively to engage the upper contacts Il. The relative positions of the contacts I l and Il is such that for any setting of the dial 23 they engage contacts I2 and I4 associated with the 7 same resistance, as atically shown in F18. 2.

'Ihe other end of brush I3 may engage the upper contacts Il of the adiacent drum 2l, tormi ing a short conductor I9 of fixed length and position between the adjacent resistance sections of theibox. Similarly, the brush 25 secured to the rear ot panel 23 forms a short path of iixed length between the second and third drums, and so on, the brush I5', connected to terminal 8 of the box, engaging the lower contacts I3 of the l'ast drum 2 0.

Brietly, rotation of knob 22 individually and selectively includes resistors of the rst group in circuit between the fixed brushes I5 and I6; rotation of knob 26 includes the resistances of the second group individually and selectively in circuit between the ilxed contacts I8 and 25;

rotation of knob 21 introduces the resistances of the third drum individually and selectively in circuit between the fixed contact 25 and the brush 28 mounted upon a supporting member 29, and, as shown in Fig. 3, engaging the contacts I4 of the two end drum's, etc. The arrangement of the remaining brushes and drums is believed apparent from the foregoing description.

The number of drums or decades`in series, or parallel, etc., can be increased toany number desired to aiford any range of resistance values.

Preferably, as shown most clearly in Figs. 8

and 9, the brushes I6, '25, etc., are so constructed that the circuit is not broken during the transition from one resistor to the next even though the surfaces of the drum contacts I4 are or become irregular, or are not exactly in the same. plane. More specifically, each brush comprises two superposed elements, as Ilia and IIib, whose contact engaging faces or tips T and Ti may be of somewhat greater extent than the distance between adjacent drum-contacts I4, and the elements IBa and IIib are of different length so that their tips are staggered. Therefore, during the change in' setting from one resistor to the next, one or the other of the tips T, T1 is in engagement with a contact I4. The contact arrangement is of particular utility in my one-impedance-per-step box to ensure that the circuit shall be continuous during the change from one resistor to the next.

The inventionmay also be applied'to other types of resistance boxes, for example, attenuator boxes, in order that for each diiferent setting, a diiferent resistance section, usually of several resistors. will be selectively included in the oircuit, to the exclusion of all other sections of the box.

In the attenuator box shown in Fig. 6, certain of the sections, as sections I to 5, are of the H type, while the remainder are of the O type. As in the prior modification, al1 of the resistances of the several sections are mounted upon a movable support and the terminal contacts of the sections are arranged to engage the fixed brushes or contacts II5, IIB, |25, and II8. For each different setting of the box, only the resistances of a single section are included in the path between the input terminals and the output terminals of the box. 'Ihe length and position of the paths between the terminals remains constant, and there is no inductance or capacity variation introduced by the change in settings.

The construction shown in Fig. 7 is suitable for the attenuator arrangements shown in Fig. 6. Upon the upper end of the terminal drum 20 are mounted two sets of contacts 3 I, 32, the outer row of which is adapted successively to engage fixed contact I I 5 and the inner row adapted successively to engage fixed contact I I6. Both of the fixed contacts may be secured to and mounted upon the support |24 secured to and extending from the rear face of panel |23. The lfixed contacts or brushes |25 and |28 similarly cooperate with two rows of contacts 30 and 33 arranged at the lower end of the drum. As the drum is rotated about f5 its axis, by a' suitable knob, or the like, the different attenuator sections are individually and selectively interposed between the input brushes II5, IIG and the output brushes I25 and II8, as

more clearly shown in Fig. 9. Each resistance section is included in circuit to the exclusion oi' all other sections, and the attenuation is due solely to the resistance of the section, and not to inductance or capacity effects introduced by the change in setting.

Itis to be understood that instead of resistances, inductances or condensers may be mounted upon the drum or drums of the box so that for different settings of the box, condensers or inductances will be individually included in ciri cuit by conductors of ilxed length and position. Further in a multiple decade box, the connections may be such that the selected impedance of one decade may be placed in parallel to theselected impedance of another decade.

For brevity in the appended claims, the term resistance section is `used to mean either a single resistance, or a group of resistances connected to each other but independent of the remaining resistances.

What I claim is:

1. A resistance box suited for precision measurements at high frequencies of alternating current comprising a pair of rotatable supports,

a group of resistors mounted on each of said sup- 35 ports with their axes substantially parallel with the axis of rotation of the support, a plurality of contacts at one end of each of said supports individually connected to corresponding terminals of the resistors, a second set of contacts at the 40 other end of each of said supports individually connected to the other terminal of the resistors, stationary brushes each adapted successively to engage the contacts at one end of one of said supports, and a stationary brush whose opposite ends successively engage the corresponding contacts at the other ends of said supports.

2. A resistance box suited for precision measurements at high frequencies of alternating current comprising a plurality of units each comprising a rotatable drum, a series of angularly spaced contacts at each end of the drum, and a group of resistors each having a terminal connected to a contact at one end of the drum and another terminal connected to an individual contact at the other end of the drum, terminals for said box, and an individual connection system of xed configuration and position for including in series between said terminals a selected resistor of each of said groups comprising fixed brushes connected to said terminals and adapted successively to engage the spaced contacts at one end of drums adjacent thereto, and fixed brush structure whose opposite ends successively engage the contacts at one end of adjacent drums.

3. A resistance box suited for precision measurements at high frequencies of alternating current comprising two fixed contacts, a xed brush having each of its ends disposed in alignment with one of said contacts, a structure carrying a plurality of resistors and movable to interpose one of said resistors between one of said ilxed contacts and the corresponding end of said brush, and a second structure carrying a plurality of resistors and movable to interpose one of said resistors between the other xed contact and the other end oi `said. brush, whereby the resistance in circuit between said ilxed contacts may be varied without change of the circuit coniiguration.

4. An impedance box comprising a rotatable support. a group oi impedances carried by said support, a group of contacts carried by said support and each connected to one of said impedances, brush structure for selectively including said im pedances in; circuit comprising superposed elements terminating in nested U-shaped tips whose contact-engaging faces are greater in extent than the spacing between adjacent contacts and spaced in the direction of relative motion of said contacts.

5. A resistancebox suited for precision measurements at high frequencies of alternating current comprising a plurality of movable supports, a group of individual resistance sections, conductors oi fixed length and position within said box, means individual to said supports for moving them to include in circuit with said conductors an individual section of each of said groups, and means cooperating with said support-moving means to indicate externally lof the box whichl of said sections are in circuit.

6. A resistance box suited for precision measurements at high frequencies o! alternating current comprising a plurality of movable supports, a group of resistors carried by each of said supports,.conductors of xed length and position 5 within said box, means individual to said supports for moving them to include in circuit with said conductors one resistor of each o! said groups, and means cooperating with said support-moving means' for indicating externally of 10 the box which of said resistors are in circuit.

7. A precision variable resistance device for -high frequency alternating current measurements comprising a plurality of movable supports, a group of units carried by each oi said supl5 ports, the units of each group having different resistance but substantially equal spurious resistance, and means for varying the resistance of the device while maintaining constant its spurious impedance comprising conductors of iixed length 20 and position, and means for moving each of said supports to include individually a unit from each of said groups in circuit with said conductors.

f LEO BEHR. 25

CERTIFICATE or CORRECTION.

mehr Nu. 2,015,651.v i t october 1, 1935.

Leo BEER.

It is herebyeertifed that error appears in the printed specification of thel above n umheredpatent requiring correction asV follows: Page 3, second column, line 54. claim 2, for "n" read anfindividual; line 57, claim 2, for "an individual" read a; page 4, second column, line 17-18, claim 7, for "resistance" readl reactance; line 20, for "impedance" read reactance; and that the said Letters Patent should be read with these Corrections therein that the same may conform to the record of the ease in the PatentOfiice. Y.

Signed and sealed this 10th day oLDecember, A. D. 1935.

Leslie Frazer (Seal) i Acting Commissioner oi Patents. I

l CERTIFICATE or CORRECTION. Parent No. 2,015,651.. I october 1, 193s.

LEO BEER.

It is hereby certified that error appears in the printed specification of 'the above numbered'patent requiring correction as. follows: Page 3, second column, line S4, claim 2, for '-'zi" read anrindividual; line 5 7, claim 2, for "an individual" read a; page 4, second column, line 17-18, claim 7, for "resistance" read' reactanee; line 20, for "Impedance" read reactance; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent4 Office.

Signed and sealed this 10th day ofDecember, A. D. 1935.

Leslie Frazer (Seal) I Acting Commissioner of Patents.. 

